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Portscan detection daemon

PortSentry has the ability to detect portscans(including stealth scans) on
the network interfaces of your machine. Upon alarm it can block the
attacker via hosts.deny, dropped route or firewall rule. It is part of the
Abacus program suite.

Note: If you have no idea what a port/stealth scan is, It's recommended to
have a look at http://sf.net/projects/sentrytools/ before installing this
package. Otherwise you might easily block hosts you'd better not (e.g. your
NFS-server, name-server, etc.).